text:
Has Air Traffic Control ever had to talk a first time pilot through a landing due to an emergency?
This question deals with the plausibility of the Hollywood cliché of a passenger on a commercial jetliner landing a passenger plane.
My question is related but broader, and less hypothetical:
Has anyone ever found themselves in an emergency with no piloting experience (e.g., the amateur pilot of a small plane dies or is incapacitated, and the only other person in the plane has never flown anything before) and been talked down by Air Traffic Control?

http://abcnews.go.com/US/wisconsin-wife-80-lands-plane-dying-pilot-husband/story?id=16062534

@abelenky Was going to post that one, happened very near to where I live, although the wife did have some pilot training many years ago.

Here's another with a private pilot with low hours landing a largish twin engine aircraft he was unfamiliar with... https://www.youtube.com/watch?v=aetmIch34M4

Lots of examples of this - especially if we're talking about single engine private aircraft.  Have you done a search? (For example there was one widely reported here in the UK, I think about two years ago. If I recall correctly he was a non-pilot and it was dark by the time he landed, though a lit runway was used.)

Here's the one I mentioned - October 2013: http://www.bbc.co.uk/news/uk-england-humber-24450534

That actually happened to one of my co-workers, although there was no time to contact ATC and he didn't exactly land the plane. The pilot had a stroke mid-flight. He passed out during final, collapsed into the yoke. My friend was able to get him off the yoke, and abort the landing. The pilot woke up briefly, lined them up with a nearby pasture then passed out again. He did his best, but he hit too hard, stuck the prop in the ground and flipped the plane. He and his wife walked away with only bumps and bruises. The pilot died from the stroke.

Related: Boy trapped in refrigerator eats own foot

The pilot in the video did have prior flying experience. However, flying a new aircraft without an instructor is a terrifying experience. A non-pilot would have been able to land the thing but with a sharp mind, nerves of steel and a bit of luck. I remember how nervous I was on my first solo landing and I was trained. There are some truly amazing people out there for sure.

The answer is yes.  One example was on April 2, 2012.  Helen Collins, an 80-year old Grandmother was forced to take the controls of her husband's CE-414 and land it at Door Country Cherryland Airport (KSUE) after her husband had a massive heart attack at the controls and died.  She managed to circle the airfield and call 911 on a cell phone.  The emergency dispatcher contacted the FAA who got in touch with ATC.  A KSUE based private pilot named Robert Yuksonavich took off and joined her in formation, talking her through what was about to happen and flew her wing on several practice approaches prior to the landing attempt.  On landing the Cessna 414 skidded off the runway and came to a stop in the grass, breaking the nose gear off.  Helen survived with only minor injuries; her husband was pronounced dead on the scene.
A brief piece by the CBS morning news featuring commentary by Capt Chesly Sullenberger about the incident.

This is the full ATC of the accident

She sounds amazingly calm. Massive kudos to her for keeping a level head.

That was an amazing achievement, especially considering Mrs Collins had just watched her husband die in the seat next to her, the person she'd shared her life with.  I can't imagine how devastating that was, then to have to land the aircraft - I think that would tax even an experienced GA pilot.  However her son stated that she had taken some flying lessons in the past and flown solo in a single engine aircraft, albeit over 30 years before this incident

In 2009 the pilot of a Kingair 200 (Registration No. N559DW) fell unconscious and died during flight while flying over South Florida, and a passenger took control and landed at Fort Myers International Airport. There was another incident in the UK where a passenger landed a Cessna 172 after the pilot had a heart attack, but I do not have a source for that.

http://www.dailymail.co.uk/news/article-2451546/Grandad-John-Wildey-landed-plane-pilot-collapsed-Hero-I-just-held-joystick.html?ico=ushome%5Emostread

John Wildey was in a Cessna 172 when the pilot became incapacitated and was successfully talked down by ATC in October 2013. He had worked as a clerk for the RAF and been on several GA flights, but had not had any flight training.
His story was featured in The Daily Mail and became an episode of Mayday.  There also happens to be a Wikipedia page on this subject.

In all fairness, much of the talking down was done by a flight instructor, not ATC. The flight instructor just happened to use ATC facilities and probably had much help from the fact that ATC cleared the airspace around a pretty large airport in response to the initial mayday call.

A totally inexperienced passenger of a Cessna 208 from the Bahamas successfully landed in Palm Beach on May 10, 2022 with guidance from a controller who is also a flight instructor.
https://www.wpbf.com/article/florida-pbia-lands-safe-passenger-pilot-private-plane/39957179#
https://www.washingtonpost.com/nation/2022/05/11/palm-beach-international-passenger-landing/
https://flightaware.com/live/flight/N333LD

As many others have said, there was, most notably when a man in his seventies died while flying his friend back to their home airport in the west parish of London, when night fell and the first time Cessna 172 pilot became nervous and was thankfully able to land the aircraft at a local international airport.


text:
What is the best reliable weight/power ration achieved by a pre-1911 plane?
From the article "DAYTON BOYS EMULATE GREAT SANTOS-DUMONT" that appeared on December 18, 1903, it can be calculated that the apparatus, which took off one day before, flew 700 lb / 16 hp = 19.84 kg / hp.
Are there planes, built by others, before say 1911, that came close to this performance?
1903-12-18, “Dayton Boys Emulate Great Santos-Dumont”, The Dayton Daily News, Ohio, US, December 18, 1903, p. 8.

DAYTON BOYS EMULATE GREAT SANTOS-DUMONT
TESTS HELD IN SECRET ON THE CAROLINA COAST.
Kitty Hawk, N. C., Dec. 17. We have made four successful flights this
morning, all against a 21-mile wind. We started from the level, with
engine power alone. Our average speed through the air was 31 miles.
Our longest time in the air was 57 seconds.  ORVILLE WRIGHT.
The above telegram was received by Bishop Milton Wright of this city last
evening from his sons Wilbur and Orville Wright, who have been for
several months at Kitty Hawk on the coast of North Carolina
experimenting with a flying machine, announcing the complete success
of their invention which they call the “Wright Flyer.”  Thursday
morning they made four successful flights against a wind of 21 miles
an hour, starting from a level and going at the rate of ten miles per
hour, which would be equal to a rate of 31 miles per hour in a calm.
No power was used in starting the flight except two aerial propellers
driven by a powerful but light gasoline engine. The landings were all
made easily and without injury to either the machine or operator. The
Wright brothers have been carrying on experiments for the past six or
seven years, and have built three previous machines for gliding by
means of which they attained proficiency in the control of their
apparatus. This year is the first time they have attempted to drive
their machine by power.
THE WRIGHT FLYER. The “Wright Flyer” is a true flying machine. It has no gas bag or balloon attachments of any kind, but is supported by
a pair of aero-curves or wings, having an area or 510 square feet. It
measures a little more than 40 feet from tip to tip and the extreme
fore and aft dimension is about 20 feet. The weight, including the
body of the aviator, is slightly over 700 pounds. The machine is
driven by a pair of aerial screw propellers placed just behind the
main wings. The power is supplied by a gasoline motor designed and
built by the Messrs. Wright in their own shop. It is of the four-cycle
type and has four cylinders. The pistons are four inches in diameter,
and have a four-inch stroke. At the speed of 1200 revolutions a minute
the engine develops 16 brake horse power, with a consumption of
little less than 10 pounds of gasoline per hour. The weight, including
carburetter and fly-wheel, is 152 pounds. The wings, though apparently
very light, have been tested to more than six times the regular load,
and it is claimed for the entire structure that it is a practical
machine, capable of withstanding the shock of repeated landings, not a
mere toy which must be entirely rebuilt after each flight.
The dispatch from Norfolk, Va., published in the morning dailies
contains much that is inaccurate and was based on imperfect or
unauthentic information. The credit for the success of the “Wright
Flyer” is jointly due to both Wilbur and Orville as the invention has
been the result of their joint labours.

source: Stack Exchange

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text:
How will a damaged logbook affect pursuing a career as a professional pilot?
I recently damaged my log book by putting it in the washer machine. I have picked through all the pieces and saved what little legible logs are left. I have requested a copy of my 8710 from the FAA. With sudden funding I have decided to pursue a career as a professional pilot. I have contacted the owner of the previous aircraft flown and requested copies of the aircraft logs. With all the information, I plan on reconstructing my log book. My question is, with my 8710 proving that I did receive my license, I also had an additional 40-50 hours of flight time that I would like to put in my log book. Should I expect any future hiccups with additional ratings and employment as a pilot? What else could I do to make this process smoother?

Related: http://aviation.stackexchange.com/questions/767/how-does-a-pilot-replace-their-logbook-if-it-has-been-lost

Well, you've already covered the "What to do if you've lost your logbook" bit.  The remaining bit of your question is a little trickier to answer, but not as bad as you might think.
The good part about renting is that as you've noted you can get the rental records & that gives you a good start on at least the hours portion of reconstructing your logs. The Day/Night, Instrument/Simulated Instrument, and destinations (Cross-Country) times may be a bit harder to reconstruct, but with a little luck you'll be able to piece that together too.

Will you have trouble with additional ratings? No.
This is the tricky bit though: You will need logs that prove that you meet the aeronautical experience requirements for any new rating you're pursuing, so if you're working through your instrument/commercial/multi and you just washed the records of that training time, cross-country time, etc. you're going to need to either reconstruct that (from rental records & landing fee receipts), fly the hours over again, or some combination of the two.
The same goes for any endorsements you have (tailwheel, high-performance, etc.) - you'll need to get re-endorsed to have a record of those (and if you show up to your next practical test in an aircraft that requires an endorsement the examiner may ask to see that endorsement).
Will you have trouble with employment? I doubt it.
Your certificates are your major qualification, and you'll have those (subject to reconstructing the requisite aeronautical experience entries like I mentioned above).
You'll want to reconstruct as much of your logs as possible because all that experience should count when you go sit down with an employer, but if you've "only" washed ~50 hours you'll make up most or all of that experience in your future training.

A few related questions you might want to look at:

What is the best way to keep logbook entries backed up?
(For my part: I scan each page once I've filled it.)
Are there any best practices for transitioning from paper logbooks to electronic logbooks?
(if you decide to go that route)

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text:
Why aren't airliner windows aligned with their seats?
On a recent airline flight, I noticed that the spacing of the plane's windows was different from that of the passenger seats: some of the rows had a window directly in line with them but for others, the windows were partially obscured by the seatbacks in front or behind.
Is there a reason that the windows aren't manufactured with a regular spacing to align with the seat rows? Does it have to do with the fact that different airlines may be using different seating configurations for the same model of aircraft?

Your last sentence is the answer to the question: it's the seats that don't line up with the windows, not the other way around.

Duplicatish: http://ux.stackexchange.com/q/73273

If windows were spaced the same as seats, then getting them to line up perfectly may end up costing the airline a row of seats.  Not every aircraft is outfitted the same way, and an efficient layout might place windows such that nobody has a window directly adjacent to their seat - fixing this might require sacrificing the efficiency of the interior layout.  However, if the spacing of windows does not match the spacing of seats (windows and rows are not sync'd), then you can maximize the number of seats which have an acceptable proximity to a window without sacrificing layout efficiency.

Because the pitch of the frames is not related to the pitch of the seats. The windows placement have to cut through as few frames as possible.

You can see it in the following picture, noting that frames are "highlighted" by the vertical riveting

Image source
It is even more evident in the picture contained in this answer on UX.SE:

Because the aircraft is designed once, but "decorated" by several airlines in several different manners. For example, an A320 or a B757 will have the windows always in the same locations, but depending on the airline that is using it, it will most probably have the seats in slightly different locations.

Because airliners skimp on spacing and steal your leg room to squeeze in another row of seats.
So why not design the plane with the eventual seat spacing in mind? Airliners can decide their own spacing so there would always be planes where they won't match up.
Also there has to be a certain amount of hull to keep the strength needed for the fuselage to work as it needs to.
So the designers need to make a compromise on number of seats and strength of the hull.

I'd add that airlines may move seats as they reconfigure the aircraft. This might happen when changing the configuration of seats in the cabin, e.g. adding or removing business class seats or converting economy seats to premium economy. Or installing new lie-flat business seats. Or squeezing the rows closer together and adding more seats. Or setting up a new layout when the aircraft is sold or leased to a new operator. Even if the rows mainly lined up with the windows when the aircraft was new, there will be some drift as an aircraft goes through many configuration changes in its lifetime.

So the designers need to make a compromise on number of seats and strength of the hull. note that those are often two distinct and separate sets of designers.

Last time I flew (on Easyjet) I noticed that there are about 3 windows for every 2 rows of seats. While I don't disagree with your first statement, the only options open to easyjet would be to reduce the spacing another 33% so that everyone has 1 window, or increase it so that everyone has two (and the latter kind of misses the point.) It would seem that for structural reasons the window pitch is often less than any practical seat arrangement.

Because airliners skimp on spacing and steal your leg room to squeeze in another row of seats But isn't there some regulation on the minimum gap between seats, especially on safety grounds?

@Firee: no, there isn't  (for now, at least). Regulations are phrased in terms of "seconds needed to evacuate the aircraft".

An aircraft is delivered in a very basic configuration; no seats, and windows running the entire length of the aircraft. The airline is then tasked with choosing and installing their specific makes of not only seats, but also galleys and sometimes lavatories. All of these must fit into that initial basic aircraft, meaning that this layout may not perfectly fit into the "mold" of the original aircraft. This means that there could be some misalignments because the seats could be of different spacing or size than the windows were originally designed for. This system of configuration is better for both the aircraft manufacturer (because it is cheaper to build one basic airframe as opposed to hundreds of slightly tweaked ones) and the airline (because they are able to customize the interior to their liking).

Aircraft are usually furnished by the manufacturer. But you are right that the airline chooses how it should be done and that each chooses differently.

When an aircraft is designed, the hull is designed in terms of 'frames' which actually support the main hull of the fuselage. Some air-crafts have provision to include more frames (upto some limit) to increase the length of the plane if later modifications are required. Now, the windows are placed in a regular spacing depending upon the length of each frame on the hull (so basically windows are spaced to remain in between the frames), and this pattern is determined after loads simulation on the aircraft's hull.
Once designed, the insides of the airplane can be modified at will (as this does not determine the 'safety' of the aircraft much), and this is when airliners try to persuade the manufacturer to put seats in a fashion that they can get the desired revenue. This independence in the hull frame spacing and the seat spacing is why the windows are not aligned with the seats.

The seats are attached to rails underneath the floor which run along the aircraft fuselage. They can be spaced any way the airline prefers, and this position does not have anything common with the window positioning.
You can buy the aircraft seat rails here, and also see how they look like.

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text:
Do airplanes have a maximum headwind gust component when landing?
An aircraft POH generally gives a maximum demonstrated crosswind component but I have not seen any indication with regards to maximum gust component.
I understand that one is supposed to increase the approach speed by half the gust component but I'm not sure if there is a limit to the gust component above which operation becomes unsafe.
Example: imagine a GA aircraft with a typical takeoff speed of 60 kts, approach speed of 70 kts. Runway is 27, wind is 270 degrees at 20 kts. What would be the maximum gust speed along the runway to operate in safe conditions?
Note: this other question is similar but focuses on crosswind component.

Ya, the maximum headwind component can not exceed VNE!

@jwzumwalt if the wind really is 100 KT and you land at 100 KT, I suspect the turbulence and wind shear will probably put you upside down before you touch down!

This is really not to far fetched. Alaska often has high steady winds for many days at a time - 40-60mph is not uncommon. 3 or 4 days long is normal but 5-7 days is not rare.

This is quite similar to the linked question but does have some differences. Gust components while reported differently are still part of the crosswind component. In other words a reporting like

Rwy 03, wind 090/25 gusting 40 kts

Is another way of saying the wind is holding at 25Kts and could be as high as 40Kts at times while you are on approach. In other words you may have a 25kt cross wind on touch down or you may have a 40Kt cross wind. Generally speaking gusts occur in the direction of the wind. In some cases gusty days also come with shifty wind conditions which add for a whole new element of fun.
The maximum gust component is not quoted in the POH because there are to many inconsistent factors that may effect it. The main one being the duration of the gust. A short gust well in excess of your crosswind maximum may be recoverable while long gusts in excess of your crosswind maximum may be unsafe. Strictly speaking a loose limit would look something like
(maximum reported crosswind) + (gust wind * gust time) > cross wind component
but gust time is not reported and thats a lot of trig to crunch on the fly so no limit is typically given. I have landed a Piper Archer in conditions gusting in excess of 40Kts safely but the wind was no more than 20 degrees off the runway.
Eventually you just run out of control surface...

Example: imagine a GA aircraft with a typical takeoff speed of 60 kts,
  approach speed of 70 kts. Runway is 27, wind is 270 degrees at 20 kts.
  What would be the maximum gust speed along the runway to operate in
  safe conditions?

In this scenario, with the wind down the runway, your maximum gust component is 50Kts. Above that and at your 70kts approach speed you would be moving backwards across the ground making it quite hard to reach the runway and potentially difficult to land the airplane if a gust were to occur at a low altitude. Most trainer aircraft like the one you describe cant do better than ~120Kts so 100Kts is about your gust limit otherwise its going to be hard to get to the runway.
NOTE: Gusty conditions (in my experience) are also generally accompanied by turbulence and in bad cases, wind shear. In many instances all of these factors combined can make an approach unsafe or un-flyable.
There is also an interesting discussion on it here.

Your approach speed doesn't matter. If the wind is 100mph and my cruise speed is 100mph then I can do a no flap vertical landing like a helicopter.

@jwzumwalt very true, I have edited to reflect.

Thank you for your answer - the bit I'm really interested in is your note at the end: yes in theory I could land in 27020G90KT with a 110KT approach speed but I suspect the turbulence generated by such wind and the high speed will make the approach and landing quite rough, if not fatal on a small aircraft. And the passengers will probably be puking all over the place :-) So I asked this question to try to find out what a reasonable limit is. If you could add a line or two on this it would be great.

@jwzumwalt What happens in those conditions the moment you shut down your engine(s), or even reduce power from what's needed to maintain a 100 mph cruise?

@Kjörling  This is really not to far fetched. Alaska often has high steady winds for many days at a time - 40-60mph is not uncommon. There are occasional accidents where a Cub or Luscomb with a 80mph cruise run out of gas trying to make headway in a 55mph wind. I landed my 172 once  in a 35mph wind - 5mph slower than my stall (1 person and low fuel). The airplane lifted off the ground twice while I taxied. Fortunately my tie down was into the wind and a couple pilots saw my predicament and tied the airplane down while I was at the throttle.

I see that you are pilots and I apologize if I say the wrong thing. With the data provided, 270/20 I agree that the headwind speed should not be greater than Vne, in fact I would say that our IAS + Headwind should not be> Vne; this increases airspeed, lift, and can cause structural failure or stall. Now, with an app of 70kts and a headwind of 20kts (and not counting gusts or shears) the GS drops to 50kts which does not favor lift or approach either; which would force us to increase power, returning again to the first problem (> aerodynamic speed ... with the danger of exceeding Vne or increasing AOA). I think the solution would be in the experience of the pilot. Now in the last question 27020G90KTS, theoretically, I would parachute.

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text:
How do I determine which radial intercept angle to use?
I'm studying for the (EASA) Instrument Rating exam and I found several questions about radial intercept angles in sample exams but nothing in the books I've studied and my flight instructor doesn't know of any specific rules either.
Based on the answers, it appears that, depending on the current and desired positions and headings, the intercept angle can be 30°, 45°, a third of the heading difference or a combination of a 90° then 45° turns.
What is the rule to determine which intercept angle to use and where is it documented?

Sample questions:

current QDM 330, HDG 060, desired QDM 350, what should be the new heading? (answer: 305 for a 45° intercept)
current QDM 120, HDG 120, desired QDR 060 after flying over VOR, what should be the heading after flying over? (answer: 040 for a 20° intercept angle, a third of the 60° heading difference between 120 and 060)

Note: I've found this document online that mentions the 45° and 90-45 techniques page 20 but it doesn't cover the other cases.
QDM: magnetic bearing to station
QDR: magnetic bearing from station

There is no definite answer to this question.
If you are far away from the beacon radials are far away from each other and thus small intercept angle would be insufficient to reach desired track in feasible time. Also, if you are, say, five miles away from the beacon intercept angle of thirty degrees would be acceptable for C172 but an overkill for transport category jet traveling at 450kts of ground speed. Then again, the mentioned jet would assume the same 30 degree intercept angle to join final track for an approach.
The question has also to do with current aircraft track in relation to the desired track. If you are for example tracking radial 150 inbound towards a beacon and you want to change to radial 180 an intercept angle of 30 degrees would keep you on radial 150 and you will not intercept the desired track until you are overhead the beacon.
The only rule seems to be that when vectoring for approach ATC gives a maximum intercept angle of 30 degrees.

The "other case" is the flyover. If you arrive at a VOR on a certain course and are requested to leave on another course. In this case you take the difference of these two courses divide it by three. Thats your intercept angle to the outbound radial. if its more than 15, its gonna be 15. (Example you arrive on a course 180° (from the north) and you're asked to leave on radial 90. Difference is 90°, divided by three is 30°, as max. is 15° you intercept the new course by 15°: means you turn left to (90°-15°=)75°.)


text:
What advantages come with the wing on the Boeing Bird of Prey?
I noticed this picture yesterday of the Bird of Prey and wondered about the wing design and what advantages would come with this unique wing design compared to other aircraft?

Source:(www.wired.com)

The Boeing Bird of Prey is indeed a unique aircraft, wahich was an internal project rather than an X-Project funded by the government. It was primarily used as a technology demonstrator for low observable technologies.
The Bird of Prey is rather unusual for a wing only aircraft (in the sense that it had no stabilizer) that it was statically stable and required no correcting inputs from a computer. This stability was achieved by the lift generated by the chines in front of the fuselage.
Here is  closer look at the chines.

Source: www.freerepublic.com
The chines produce lift by generating vortices (similar to the delta wing). Basically, the chines produce seperated vortices on either side of the fuselage, which produces lift. According to adg.stanford.edu,

Rather than reducing the lift of the wing, the leading edge vortices, increase the wing lift in a nonlinear manner. The vortex can be viewed as reducing the upper surface pressures by inducing higher velocities on the upper surface

Image from Effects of Vertical Tail and Inlet/Strake Geometry on Stability
of a Diamond-Wing Fighter Configuration by Mitchell E. Fossum et al., accessed through http://enu.kz/
To compare how the chine generates lift similar to a delta wing, it is instructive to see the vortex flow patterns on a slender (cropped) delta wing with a chine forebody.

Image from On the Nonlinear Aerodynamic and Stability Characteristics of
a Generic Chine-Forebody Slender-Wing Fighter - Configuration by Gary E. Erickson and Jay M. Brandon, NASA Technical Memorandum 89447, labeling mine.
An example of a delta wing with a chine forebody shows the two vortices being generated from the chines and the highly sweptback wing, similar to the Bird of Prey.

Image from Aerodynamic Analysis of a Generic Fighter with a Chine
Fuselage/Delta Wing Configuration Using Delayed Detached-Eddy Simulation by Tiger L. Jeans et al, accessed through calpoly.edu
The forces acting on the longitudinal axis of the aircraft look something like this (I'm not sure of the exact values or locations), leading to a statically stable aircraft.

Profile from bj-o23.deviantart.com, others own work.
The chines also help in reducing the radar cross section, according to dictionary.sensagent.com (attributed to The Radar Game by Rebecca Grant):

A standard leading edge extension joins to the fuselage at an angle while a chine is an extension of the curvature of the fuselage. Therefore chines avoid presenting corner reflectors or vertical sides to radars.

This design has since become standard in (Boeing) UAVs like X-45 etc.
Another feature is that the fuselage sides are parallel to each other, helping in reducing the Radar Cross section by reflecting the radar waves away from source.

Source: gizmodo.com
The wing design is rather unusual, a polyhedral section with dihedral (upwardly angled) inboard sections and anhedral (downwardly angled) outboard sections, which helped in stealth and stability.

Source: soldiersystems.net
The wing has a rather high sweep angle (~65 degrees) for a low speed aircraft, in order to improve stealth.

Background
Since @aeroalias' answer just reproduces what is said in Wikipedia, I will add a few more thoughts (albeit without the pictures, there are already plenty in the other answer).
First of all, the design is meant for transsonic flight, but the development was privately funded by McDonnell-Douglas and had to keep cost down. As a consequence, the demonstrator aircraft used a 3000 lb thrust class engine (Pratt & Whitney Canada JT15D5C) which is used more in military trainers and business jets than in combat aircraft. This kept the aircraft's top speed and the aerodynamic forces low, which helped in keeping the systems simple and the structure small. A production aircraft would be much bigger and use a much more powerful engine.
When McDonnell-Douglas was not selected for the ATF program in the late Eighties, they decided to put themselves on the map by building a demonstrator which should show off their low-observables expertise. They had founded their own design office for black programs, called Phantom Works, mirroring the approach of Lockheed with their Skunk Works branch.
The aircraft
Roughly half of the lift in level flight is produced by the forward fuselage. Wikipedia says it is the chines, but they only provoke the generation of a pair of vortices just like those on a delta wing, and the low pressure caused by the speeds of this vortex creates lift on the forward fuselage. In a way, the Bird of Prey is a canard aircraft.
The other half of lift is contributed by the wings, which again create vortex lift due to the high leading edge sweep angle. The dihedral of the inner wing was chosen to

get the tips up, so the winglets/fins have enough space to point downwards. Note that the tips of the winglets are cut back to provide enough clearance for rotation. This cutback angle also indicates the rotation angle and the angle of attack at low speed.
provide a negative rolling moment due to sideslip, reducing the winglets' effect.

Regular, upward-pointing winglets have a number of disadvantages, one of them their induction of lift on the wing when their rudders are deflected, which produces an undesired rolling moment and prevents rolling the aircraft into the intended turn. Downward-pointing winglets helped to make the aircraft flyable without computer control, because they help with

directional stability, possible by their rear position due to the high wing sweep angle
rolling moment with rudder deflection, rolling the aircraft into the direction of the turn commanded by the rudders.

The high sweep angle was chosen for the high-subsonic flight regime of any production version and for deflecting the returning radar signal away from the sender. The Bird of Prey is a two-lobe design (see this answer for more details)

How is this vortex lift created? I have never heard of it before.  The only lift I have heard of is the high and low pressures on the wing. Do delta wings have a different way to produce lift?

@Ethan: See this answer for an explanation. Lift is still produced by low pressure above the wing, but the mechanism to achieve this low pressure is different in delta wings at high angle of attack.

The trailing edge eliminated pressure differences across the wingspan, therefore eliminating contrails at lower altitudes which have water vapor that leads to their formation. This enables use of this stealth aircraft during daylight hours.

Can you support this answer, which differs wildly from the other 2 answers posted, with any sources? Problematic contrails tend to come from the engines at high altitude; the common scenarios for contrails from the wing at low altitude, pulling G's or landing configuration + high humidity, seem like not the cases where stealth would be compromised.


text:
What effect does cold weather have on helicopter performance?
Recently I was flying around my RC helicopter on a 40 $^\circ$F (4$^\circ$C).
At some point in the flight the rotor just stopped moving in mid-air and then I regained control back at ground level. I never had any issues before in warmer weather, so I assume it's a cold-weather related issue with this model.
This leaves me with two questions:

Could something like this  happen in a full-size helicopter like a Robinson R22?
What effect does temperature have on helicopter performance in general?

The rotors stopping abruptly is NOT something regular helicopters will suffer from normally, I'm pretty sure of that... You may have a problem with battery performance at low temperature, or if you were outdoors and the controller is infra-red, the signal may have been interrupted.  Or there could be any sort of random glitch in the electronics.

@Ethan If I were you, I would ask this questions on Electronics.SE since I'm pretty sure it's a battery- or electronics-related problem.

@Ethan, I cleaned up the question to make it more to the point, and aviation related. Feel free to rolll back the changes if you're not satisfied with the result

First you should try to understand if the result was the rotor being blocked, or the engine that had stopped. I doubt reasons could be compared, a RC motor is electric (I assume) and R22 is a piston engine or a turbine. What could stop one will likely no affect the other. If this is the rotor itself, due to a lubrication or thermal contraction cause, the two mechanisms are not at all similarly built and are not affected the same way (my assumption). Don't you have the temperature operating range in the manual, by chance?

I think (also given the two closevotes on this question), you might want to ask these questions in a different manner: instead of "I played with my quadcopter and then X happened", your question might be better received by the community when you ask "I was wondering whether X could happen; here's what I found so far in my own research". To me, it sounds like you post whatever your quadcopter does without any prior research (whether this is true or not).

Cold weather definitely affects the performance of helicopters (as it does all aircraft), because it affects the density of the air they move through.
I will ignore icing so let's assume dry air.
The lift equation is:
$$ L = \frac{1}{2} \rho V^2 S C_L $$

$L$ = Lift

$\rho$ = density of the air.
$V$ = velocity of the aerofoil (rotor)
$S$ = the wing area of the rotor
$C_L$ = Coefficient of lift , which is determined by the type of rotor and angle of attack.

As you can see, lift is proportional to the density of the air and since density at any given altitude is inversely proportional to the temperature, the colder the air, the more dense it is.  So, for a given rotor, spinning at a given speed at a given altitude, the colder the air, the more lift is produced.
Engines also operate more efficiently when the air is colder since the density is higher and the charge of air/fuel introduced into the engine is greater. (See here for more information.
 Of course, this does not apply to your RCH apart from some effects on the efficiency of the batteries and motors (which may perform more poorly at lower temperatures).
So in general, the colder it is, the higher the performance.  In most helicopters, the difference in performance on a cold, dry winter day is very noticeable and much better than on a humid, hot summer day.
It's also worth understanding the properties of air.  The colder the air, the lower the dewpoint so the less moisture it can contain.  Moisture in the air also lowers peformance.

The problems you encountered with your RC model aren't problems with helicopters in general.
Colder weather does not make you more susceptible to vortex ring state (aside from some minor side effects): You would still only descend into the vortex if you did not pilot the helicopter correctly. (Note that you cannot enter vortex ring state with the rotors stopped since they are not producing a vortex.  Also note that on a real helicopter, if the rotors ever stop, they will not start again no matter what you do.)
BTW, I'm being pedantic, but helicopters do not have propellors.  Technically, they are the same thing, but asking a helicopter pilot about the propellor annoys them, even if they don't show it.  It's as bad a calling a helicopter a "chopper".  Only Arnie and Chuck get to call helicopters "choppers".  And that's only because helicopter pilots are universally weedy and would get stomped by Arnie or Chuck if they argued.

I tweaked this a little to line it up with the edits to the question, if I butchered any of the helicopter-specific stuff feel free to roll it back :)

What is vortex ring state. Is that the wake vortex coming from the propellers. The helicopter started to shake like crazy when I descended it into its own downwash too fast

@Ethan Shaking is one of the symptoms of VRS.  See this

First, I don't think cold weather had anything to do the stopping of rotor. Probably a malfunction.
However, Helicopters do face a number of problems during flight in cold conditions

Ice accumulation in leading edges- Very few helicopters have leading edge anti-icing systems and are certified for flying in cold conditions (like North Sea).
The possibility of misting in windshield.
Whiteout conditions, usually while taking off and landing.
Ice/Snow ingestion in engines.
Icing and blockage of fuel lines etc.

Most of these have to do with icing and not with cold conditions as such. In fact, as Lift is directly proportional to density (L $\propto$ $\rho$), helicopters have better performance in cold weather (as air density is higher) and rotors produce more lift. Engine power is also usually increased. It is in hot-high conditions that helicopter lift production is critically affected.

I'm adding this as the question has been cleaned up and changed a bit (effect of temperature in helicopter performance).
Temperature does affect the performance of the helicopter, as the density varies with temperature. The performance of the helicopter depends on three main atmospheric parameters (assuming all other helicopter parameters are equal):

Density altitude (air density)
Gross weight
Wind velocity (during takeoff, hovering, and landing)

The lift produced by the Helicopter blades is given by,
$L \ = \ \frac{1}{2} C_{L} \rho  V^{2} S$
Now, in helicopters, $S$, the blade area, $V$, due to rotor speed are fixed. So, as the density varies (due to altitude or weather), the lift coefficient $C_{L}$ has to be adjusted by varying the angle of attack of the blade.
As the density decreases (due to hotter temperature or higher altitude), the required angle of attack (for producing same lift) increases. However, the maximum angle is limited by the blade stall angle. The net result is that the helicopter's ability to hover (or fly) at a particular altitude (or weight) is reduced due to change in temperature.
The following image shows the hover chart for CH-47D. It is obvious from the chart that the hover performance is significantly improved as the temperatures fall, at least till 0$^{\circ}$

Source: globalsecurity.org

On a model, yes, cold weather might have something to do with propellers stopping: batteries don't like cold. Though +4°C is not that cold.

@JanHudec I'm assuming that the batteries are LiPo or Li-ion. They should usually operate at 4°C, though I can't be sure. Also, Ethan says that he regained control in a few seconds. I'm not sure if its battery problem

Indeed, 4°C does not sound like the batteries should already have problems.

Besides the aerodynamic aspects of cold temperatues, there is another aspect concerning the thermodynamics of the engine. Power and efficacy are better the colder the air is.
However, in your RC model, maybe the clearance between some moving components is too small so they gall as the material expands at higher temperatures.


text:
Why is cabin pressure increased above ambient pressure on the ground?
As far as I know at least some (e.g. some Bombardier Dash-8 series) airplanes during takeoff may increase cabin pressure above air pressure on the ground, before starting to decrease it. In one case pressure was increased to around 760 mm Hg (1013 hPa) when pressure on ground was around 730 mm Hg (973 hPa).
Is this increase of cabin pressure desired for some reason or is it just a side effect of something? And why does it happen?

Would it be to ensure inflow of oxygen? How long will the oxygen inside a sealed cabin last with 100-400 people consuming it?

Short answer
The purpose of pre-pressurization is to avoid discomfort for passengers and crews during takeoff and landing.
By storing air in the cabin before the aircraft rotation or landing, the pressurization controller is able to smooth out:

The variations of bleed air supply when the engines deliver the highest thrust and need all available air.
The variations in both air supply and air release, due to the aircraft leaving or entering the ground effect. In particular this ground effect would force air into the cabin through the forward outflow valve when the angle of attack is increased for takeoff rotation or landing flare out.

However pre-pressurization involves relatively limited pressure differentials.

236 ft for a B737-NG (9 hPa).
140 ft for a Dash8-200-300 (5 hPa).

The case you mention seems ways higher with 40 hPa.

Pressurization system
The source of cabin air for the aircraft is commonly bleed air from the engine compressor stages. Air is sent to packs for conditioning and enters the cabin (see a more complete description in this answer):

(B737-800, source)
Bleed air pumped into the cabin then escapes it by one or two configurable exits, the outflow valves:

Outflow valve (and near to it the safety valve) on a Boeing 737-NG. Source
Cabin pressure is obtained by controlling the rates the air enters and leaves the cabin. This is indeed the job of the pressurization controller. This job is more difficult at takeoff and landing:

Air taken from the compressor is not used for the production of thrust, which can be a problem when thrust is critically required, e.g. for takeoff roll and rotation, or go-around. For these sequences, bleed air may be not available, or may be delivered at reduced or unstable rates. Normal bleed air delivery capability is restored after takeoff when the engines need less air.
The ambient pressure around the aircraft is subject to variations when the aircraft rotates and leaves the benefit of the ground effect, and also when it enters this effect before the flare. So the differential pressure at the outflow valve varies as well as the bleed air supply, and both need to be compensated too.

Pre-pressurization
Unmanaged variation of bleed air supply or cabin air outflow create air bumps in the cabin. To avoid discomfort, on some aircraft the cabin is pre-pressurized at a pressure slightly higher than the ground ambient pressure.
The pre-pressurization occurs during the takeoff roll and shortly before landing. It creates a stock of air the pressurization controller can use to smooth out bleed air bumps.
Source: B737-800 FCOM:

The air/ground safety sensor signals whether the airplane is on the
  ground or in  the air. On the ground and at lower power settings, the
  cabin is depressurized by  driving the outflow valve to the full open
  position.
The cabin begins to pressurize on the ground at higher power settings.
  The controller modulates the outflow valve toward close, slightly
  pressurizing the cabin. This ground pressurization of the cabin makes
  the transition to pressurized flight more gradual for the passengers
  and crew, and also gives the system better response to ground effect
  pressure changes during takeoff.

This additional amount of pressure mustn't compromise the ability to quickly open the doors in case of emergency, so pressure differential is limited. On a B737-NG, the differential is 0.125 PSI (9 hPa). This is equivalent to an altitude 236 ft below the runway. The system has a fail-safe mode which automatically opens the outflow valves quickly after the aircraft has landed (unless the Ditching switch has been pressed).
After the aircraft is airborne, or has landed, the pressurization system releases air previously stocked, to recreate a smooth altitude increase.
For the sake of completeness: The cabin will not "climb" or "descend" as fast as the aircraft, and will have its ceiling at 8,000 ft to maintain oxygen pressure at a safe level to breath without a mask.

Cabin pressure vs. atmosphere pressure
Here is a representation of the pressure differential during a flight (with the takeoff/landing highlighted. Note the cabin (dashed line) "is lower" than the aircraft.

(Source)

If that was the short answer, I'm not sure I want to read the long answer!

Whew! I thought you were going to add the long version!


text:
What is the advantage of the blisk design in the new Trent XWB engine?
What advantage does the "blisk" or "bladed disk" type rotor have over the usual rotor system?

https://en.wikipedia.org/wiki/Blisk#Advantages

As @mins points out, this question shows a pretty clear lack of research.

Since the blades and rotor disc are a single piece, there are no additional fittings (bolts, screws etc) to attach the blades reducing complexity and weight.  The blade root joints are also a source of failure through manufacturing defects and fatigue cracks.
The disadvantage is that any damage to a blade requires the entire assembly to be replaced rather than a single blade.

"Module weight savings of 15% and improved aerodynamic efficiency", according to Rolls Royce.
From a manufacturing point of view, replacing literally hundreds of sub-components (individual blades, etc) with a single part has obvious advantages - though of itself, that wouldn't give the customer any benefit if it didn't also reduce weight and/or improve performance.

If something is advantageous to the manufacturer, one would expect that to lead directly to a less expensive part for the customer.

You might, I wouldn't. Products are priced according to profit, demand etc, not according to the cost to produce.

The above answers give very good technical reasons for blisks.  Less parts means less rotating weight which has a high impact on overall weight.
However, the technical benefits are not the only considerations.  As engines age eventually they are worth more in parts than as an engine and the parts are salvaged and sold.  The compressor blades can bring in a tidy sum.  However some will have erosion and other nicks that will not meet the manual limits for returning to service.  So not 100% of the parts removed from a compressor can be salvaged and sold.  There are several thousand blades in a modern high pressure compressor.  This salvage market is competition to the aero engine manufacturers.  The manufacturers would rather you buy a new part from them than a used part from someone else.
Aircraft engines follow the same economic model as razor blades.  The initial purchase may be sold at a loss but the replacement parts will be profit drivers.
A salvaged Blisk will have airfoils with erosion and or nicks and dents on the airfoils (this can happen after only a few hours of operation)  So the blisk assembly will not be able to be inspected serviceable.  The engine operator will need to send the part to the Engine Manufacturer for a proprietary repair.  This gives the engine OEM a better control on the used material on the market.
In conclusion, less weight is always good but the blisk also provides an opportunity for the engine OEMs to ensure that they sell parts and make a return on their initial investment.

Do you have much evidence to support this? I was under the impression many engines are actually rented rather than purchased so it's the OEM's loss if parts can't be refurbished. Also with the stricter and stricter rules around emissions every bit of weight that can be saved counts and 15% of a large blisk is a considerable amount of weight!

@Notts90, you are under the right impression. RR has switched to renting engines. So it is unlikely there are any Trent XWBs not owned by RR and therefore any point about competition from refurbished part resellers is moot.


text:
Do logged hours with a Sport Pilot instructor count towards a private pilot certificate?
I've my Sport Pilot licence and I've got logged hours with a sport pilot instructor that isn't a private pilot instructor.
Can I use my logged instruction hours in order to upgrade to private pilot?

No.  Sport pilot instructors are not authorized instructors with respect to private pilot certification.
See 61.109(a).
The FAA has issued a legal interpretation on the matter.  See the Kern Ink interpretation


text:
Why are the Boeing 747-8 engines on the wings spread apart?
I ask this because the Boeing 747-236 had the engines close together while the 747's have them spread apart. Does it have anything to do with the amount of thrust of the aircraft?
Look at how spread apart the engines are:

Now look at how close together those engines are on the Boeing 747-236:

As the original post on Airliners.net clearly states, the second aircraft is not a real one, but a prop for a movie.No real life Boeing has this configuration.

Short answer
It's the vastly different flow conditions from static to cruise which demand a separate placing of high-bypass-ratio jet engines. They would produce less thrust and more drag when paired.
Why were there paired engines at all?
The early jets had their engines mounted directly beneath or in the wing, and comparisons between separately mounted and twinned engines showed a slight advantage for the latter due to lower wetted surface area and less impact on the wing.
Arado built two four-engined prototypes of their Ar-234 jet, one with separate engines (V6, see directly below) and one with paired engines (V8, further down).

This twinned arrangement works well with turbojets and low-bypass-ratio turbofans. The Boeing B-47 and B-52 bombers, and among airliners the Illyushin 62 and Vickers VC-10 all had paired engines.
However, with the increasing airflow of high-bypass-ratio engines the interference between both will turn pairing into a disadvantage. In cruise, only the central stream tube flowing towards the engine will be ingested, and the rest will spill over the intake lip. Placing a second engine directly next to the first will block the flow of spilled air on that side and will increase spill flow on the opposite side. This will most likely cause massive separation there if the intake is not heavily modified, leading to noticeable drag increase. Also, the now asymmetric flow in the intake would reduce the efficiency of the fan - it needs very homogenous flow over the full cross section in today's engines.
Conversely, at low speed the engine sucks in air from all around and will face competition from a second engine, such that both will not be able to ingest as much air as when mounted separately. The consequence of pairing would be reduced thrust during take-off.
The initial disadvantage of separate engines, their collective impact on wing aerodynamics, is now greatly reduced by mounting them on pylons, so they are ahead and below the wing.

What about B-52s then?

@PeterMortensen: The B-52 had turbojets. The J-57s were more similar to the BMWs of the Arado and less sensitive to interference compared to a modern turbofan. The pairing of eight of them (initially Westinghouse J-40s) was decided very early, coming from a long experience with four-engined aircraft. I do not know whether eight individual engines were ever tested - the pairing worked in the B-47 already and was then used in the B-52, too.

https://aviation.stackexchange.com/questions/4966/what-is-the-difference-between-a-turbofan-and-a-turboprop-engine - difference between turbofan, turbojet (and turboprop)

That picture of the 747-200 you have with the engines in a pod together like a B-52 is a movie prop. http://www.airliners.net/photo/Boeing-747-236B/1070481/
All real 747s from the -100 through the -8 have their engines positioned like the first picture.
EDIT - Federico's response reminded me of another aspect of the engine position, and to answer your original question of why they're spread apart. Spacing between the 747s engines is important due to the need to maintain sufficiently clean airflow into the inlet. If the engines are too close together, they're essentially competing for the same amount of air, and at high power settings, the interaction between the two fans will disrupt their operation, potentially causing engine stall/surge. Additionally, the 747 uses thrust reversers, and the spacing between the engines helps ensure that the reverse flow from each engine doesn't impact an adjacent engine, for the same inlet distortion reason.
I assume, though I don't know the details of this, that the B-52 can have them that close together because they've shown that the inlet geometry, airflow required by each engine, whatever other magic of fluid dynamics that I know nothing about, prevents the engines from distorting each other's airflow enough to cause an issue. The 747 engines require a lot more air, and generate a lot more thrust, and are potentially more sensitive to inlet disruption than the engines on the B-52. This last paragraph is mostly a guess on my part as I'm not an aerodynamicist, but it's inferred from issues we've had to deal with on the 747 and other related airplanes.

Indeed; this prop is the fictional "Skyfleet S570" aircraft in the film Quantum of Solace. http://jamesbond.wikia.com/wiki/Skyfleet_S570

James Bond movie Casino Royale?

Here is a wikipedia article on the airplane in question. The link in this answer appears to be dead unfortunately.

In addition to what Stickhog's answer says, the fact that all 747s (and A380s) have their engines spread apart has to do with the "single point of failure" concept.
Consider for a moment the picture of the movie prop you posted, if one of the pylons suffer any kind of mechanical failure, you lose 2 engines. The same with one of the engines suffering explosive failure or uncontained fire.
Having the engines separated from each other reduces the risk that a failure that renders one engine inoperative will affect also another engine.

Then why did the B-52 use two-engine pods? Was it just that 8 engines was plenty of redundancy?

@cpast if you lose 2 engines on the same side on a 747 you will have issues with yaw control. if you lose 2 on the B52, it's like losing one on the 747.

As well as the issues relating to the aerodynamics of the engine intakes and single-point-failures, the distance of the engines from the aircraft center-line affects the vibration frequencies of the wing and hence the flutter stability margin of the aircraft. That turned out to be a significant problem on the first version of the B747, to find a compromise position which could use three different engine types from the three engine manufacturers on a common wing design.
The first commercial jet airliner design addressed those problems by "twinned engines" buried in the wing root. For pictures see https://en.wikipedia.org/wiki/De_Havilland_Comet. That design persisted in the derivative Nimrod military surveillance aircraft https://en.wikipedia.org/wiki/Hawker_Siddeley_Nimrod until its recent retirement. The problems of trying to shoehorn a modern engine into that airframe configuration, and meeting modern safety standards (as opposed to 1950s era standards) was one of the problems which resulted in the proposed upgrade of the Nimrod MR2 to the MR4 to be abandoned (The MR2s were retired from service in 2011).


text:
Why are contra-rotating jet engines so rare?
Why are contra-rotating jet engines so rare? Such a jet engine would have contra-rotating turbines, or contra-rotating compressors, or both.
In fact I cannot think of even one aircraft that has this. If some prop-engines have contra-rotators, I don't see why some jets don't have it either.
It seems to me the contra-rotating parts would be more efficient in extracting power or compressing air. There are already contra-rotating turboprobs (Tu-95), and turboprops are very similar to jet engines, so I think it is not a question of technical knowledge. Another benefit would be reduced or eliminated gyroscopic forces hampering yaw or pitch torque, which I will ask another question about.
BTW, I am considering axial-flow jet engines only. I totally forgot there used to be centrifugally compressed jet engines, which have all but disappeared.
EDIT: The question is about jets (either turbofans or turbojets), but I mentioned the Tu-95 and its turboprop engines because turboprop spools are very similar to jet engines. However, I thought those Tu-95 engines had contra-rotating everything, but it turned out only the propellers were contra-rotating, not the turbines. Nevertheless, if there was a turboprop with contra-rotating spools, I would very much like to hear about it here because I think it would have great bearing on the question of contra-rotating jet engines and their efficiency.

The P&W F119 (F-22's engine) has contra-rotating turbines and compressors. (That is, rotors: you can't have one without the other :) They claim it allows to make the engine shorter (by removing stator stages between the compressors and turbines) and improve efficiency. But I don't know how it actually fares against increased wear and mechanical losses. I guess it must be more reasonable for the top-performing and/or special military engines such as F119 or Pegasus than for longer-living civil engines.

@Zeus The wiki page for the F119 (https://en.wikipedia.org/wiki/Pratt_%26_Whitney_F119) describes it as a twin spool engine. Nowhere does it mention contra-rotating anything. Are you sure it's contra-rotating and do you have a source?

For example, right here: http://www.pw.utc.com/F119_Engine . By the way, it's direct derivative, F135 for F-35, is similar in this regard.

Many jet engines have centrifugal high pressure compressors, including modern designs that are currently in production.

@JWalters Can you name a few? Even if they are small business jet engines, I'd still like to look at them.

@DrZ214 Sure: the entire Williams Fanjet line (FJ44, etc), many Pratt & Whitney designs (JT15D, PW530, PW535, etc). I believe the Honeywell TFE731 as well. Every turbine engine I've ever operated or worked on has had the centrifugal high pressure compressor, including the PT6 line of turbo-prop engines.

If the low-speed and high-speed spool rotate in the same direction, the relative speed between them is low. Contra-rotating means you need to double the individual speeds to arrive at the relative speed, and this will increase bearing losses greatly. Both friction and wear will go up with little benefit in regular operation. A notable exception is the Rolls-Royce Pegasus engine for the Harrier jump jet.
Stators between the rotating stages ensure that air flow inside the engine is mainly in axial direction, so aerodynamically a different rotation direction between two spools will make little difference. However, contra-rotation will indeed make the stator between the last stage of the high- and the first stage of the low-pressure turbine obsolete. The video linked by @Moo cites a 10% reduction in part count as the advantage. Additionally, the first stage of the low-pressure turbine should show improved efficiency (like the second propeller of a contra-rotating propeller).

Can you explain a little more why with little benefit in regular operation.? In particular, the engines of the Tu-95 are contra-rotating turboprops, and the turbines of turboprops are very similar to jets. Or maybe those engines did not have contra-rotating spools, but just contra-rotating propellers?

@DrZ214: Yes, the NK-12 of the Tu-95 drives the propellers through a single shaft, and contra-rotation is achieved in the gearbox. Regarding little benefit: The second paragraph already discusses the lack of aerodynamic benefit, and the gyro effects are small compared to the damping and control forces available in regular flight. Only in a VTOL design would contra-rotation offer a tangible benefit.

Do contra-rotating spools within jet turbines need stators though? To me it seems two blades sweeping right next to each other will produce enough compression or extract enough torque such that stators are unnecessary.

@DrZ214 Every spool has several stages, and between each the stators re-direct the flow and convert the rotational component into a small pressure increase. Only at the transition from the last low-speed stage to the first high-speed stage would contra-rotation make a difference by making the stator there redundant.

@DrZ214, the question specifically asks about jet engines. Tu-95 is a turboprop and therefore out of scope.

@JanHudec: Considering the body of the question, I'd assume that the them "jet engine" refer to gas turbine engines in general

@slebetman, the body of the question is extremely clear in that it does not.

It should be mentioned that the high and low pressure shafts are concentric, which is why there is a bearing directly between them. It took me quite long to realize why the argument is relevant.

@JanHudec The high and low pressure shafts are not necessarily rotating at the same RPMs, correct? I think this is the terminology different spools. Maybe contra-rotating spools are not so far off from this. BTW, I'll make an edit to the post to hopefully clear up jets vs turbostuff.

Speaking of that, I wonder if the F-135-PW400 engine for the F-35B makes use of contra rotating spools or they just use asymmetrical thrust from the roll posts to counter engine torque.

@DrZ214, no, they are not rotating at the same RPMs. But when they are rotating in the same direction, the bearing between them only needs to handle the difference while when they are rotating in opposite direction, it needs to handle the sum. And that's the Peter's first argument.

@JanHudec Okay. Can you or Peter Kampf quote some typical RPMs of the spools for turbofans and propellers during cruise? I want to see if there's a major difference.

@DrZ214 It's all here

It seems to me the contra-rotating parts would be more efficient in extracting power or compressing air.

With propeller, contra-rotating propellers are the only way to stop rotation of the slip-stream. However jet engines have static guide vanes that stop the rotation almost as efficiently with a fraction of the complexity of actual contra-rotation.

Another benefit would be reduced or eliminated gyroscopic forces hampering yaw or pitch torque

These are negligible compared to the aerodynamic forces in flight. The only aircraft where it matters is the Hawker Siddeley Harrier and that does, indeed, have engine with contra-rotating spools.

Contra rotation in civil turbine engines was first introduced by Rolls Royce in the Trent 900, where its high pressure spool rotated in the opposite direction to the other spools.
http://www.prnewswire.co.uk/news-releases/rolls-royce-trent-900-engines-provide-power-for-first-a380-154499805.html
The GEnx engine family use contra rotating spools - the high pressure compressor is driven clockwise while the low pressure compressor is drive counterclockwise.

The Trent XWB engine family follow on from the Trent 900 and also use contra rotating spools - the high pressure compressor is driven clockwise, while the low pressure and intermediate pressure compressors are driven counterclockwise.
http://www.easa.europa.eu/system/files/dfu/EASA-TCDS-E.111_Rolls--Royce_plc_Trent_XWB_Series_engines-01-07022013.pdf

Interesting. Is there any word on how much more efficient this is compared to a non-contra-rotating turbofan of the same thrust?

Given that contra-rotation was only introduced very recently to the Trent line (with the 8115), what is the technical benefit of doing so?

Torque reduction DOES NOT MATTER and is NOT a factor either way. What counter rotation does for a turbofan (in the present non-geared designs) is

Increase the efficiency of the compressor by switching the 'swirl direction' between the fan & the first compressor stage. This has the effect of making the compressor close to unstallable, which means you can run WAY higher compression ratios, thus increasing efficiency.
Between the high pressure & low pressure exhaust stages, counter rotating blades extract more energy from the exhaust gases, thus delivering more torque to the bypass fan. For example the GE genx.

The real benefits of the counter rotating spool jet engines are the following:

Increase the efficiency by the increased compression ratio of the compressor, due to the counter rotation at intersection between the low and the high pressure stages. And just there. Between the last stage of the low pressure spool and the first stage of the high one, there's no need for a stator, because of the opposing orientation of the counter-rotating blades. The counter rotatating blades, according to what is rotating in the front of it (but in the opposite direction) acting like a stator.
How do you build up the pressure step on a compressor stage? You have a rotor which speeds up the airflow, and a stator, which slows it down, but meanwhile increases the pressure. So if you have a rotor with for example 10000 RPM, the stator "RPM" is zero, compared to it.
Now, if you have a counter rotating shaft, its rotor with another 10000 RPM means that the relative rotating speed is going to be 20000 RPM. So the step up will be much higher, the pressure ratio at there is increased. But only at there. At the other stages of the LP and the HP compressors has to contain a stator. This is the same rule of the turbine as well, with the same benefit.

The second benefit is not the torque reduction. This would not make sense in the jet engine world, only in the propeller engine world (piston engine or turboprop).
However, the real thing is precession reduction, which is essential for the high AoA manoeuvring. The rotating mass has its own precession, which tries to turning around the shaft of the rotating mass.
This is obvious if you have a low speed run at the runway and you hit the throttle on a propeller plane. The propeller torque is just one thing, because it would like to turn your plane around the longitudinal axis (but the gears are holding it). The precession of it would like to rudder you out from the runway.
This departure attitude depends on the rotating mass and the RPM changes. The only thing which could stabilize it is the airflow around the plane, when your speed is building up. But inside a fighter jet, (F-22 or the F-35), the rotating mass of the shafts would like to break the plane out during a extremely low speed, high angle of attack manoeuvre, because of the rotating axis and the moving vector is not pointed in the same direction.
It is a critical situation. If you check the Russian Su-30MKI...SM or the Su-35S, or Su-57 videos, you will clearly see, how the huge rudders and the TVC nozzles try to save the pilot's day, during a post stall manoeuvring, driven by the very sophisticated Fly By Wire system. And very often, you can see a lot of unwanted slides, especially at the top of these low speed manoeuvres, because of the AL-31FP or AL-41F1S engines' similar direction spools. More on this, on those aircraft, the engines are quite far away from the longitudinal axis, (unlike the F-22), so have a quite large rotating momentum (torque) as well.

And of course, due to the issue of the precession, all VTOL aircraft have counter rotating spools, because it could prevent the aircraft starting to rotate slowly around the vertical axis. And no, this is not the same phenomenon as the helicopters have, by the main rotor torque. It is not from the torque. It is the precession.

Because of the difficult flow calculation and relatively small numbers of really new engines. By other words, everyone just use their already-on-stock design to reduce the costs.

I answered to your question.

If you're talking about making contra rotating spools in an axial flow jet engine, the major advantage there would be torque reduction.  However, keep in mind that the torque applied to each spool may have a considerable disparity in in magnitude, effectively eliminating much of the benefit.
Making individual stages contra rotating would have a nightmarish complexity in terms of gearing and associated systems to drive this, in addition to a serious weight gain and reduced reliability.  For those reasons such a design was never seriously considered.
Now contra rotating fans have been considered from time to time.  Some examples which directly come to mind are the UDF aft unducted fan design by General Electric in the 1970s as an attempt to increase engine efficiency.  And SENECMA is apparently working on a contra rotating turbofan fan for increased efficiency as well as reduced IR signatures for UAVs.

the major advantage there would be torque reduction. What does "torque reduction" mean? If it's reducing the driving torque obtained from the turbine, that would reduce engine power and thus reduce efficiency. keep in mind that the torque applied to each spool may have a considerable disparity in in magnitude Why is that? If each turbine has the same number of blades, same shape, and right next to each other, how could there be considerable disparity just from rotating in opposite directions? (I may have to open a new question on how contra-rotating jets would work).

BTW, thanks for pointing out axial flow jets. I totally forgot there were centrifugally-compressed jet engines which have all but disappeared.

The NK-12 is a single shaft turboprop with a counter rotating gearbox which drives the co-axial propeller.
However, there are a few real counter rotating jet engines.
For example the P&W F119PW100 from F-22 Raptor, the F135 from the F-35 family, the Yak-141's Soyuz R-79V-300, the MiG 1.44's AL-41F and the Harrier's Pegasus engine as well.
These are twin spool engines.
There are two main big benefits:
- Higher compression ratio by the higher relative airflow speed at the meeting point of the two spools;
- Almost zero precessive force due to the balanced torque of each rotors.
This one especially good for the VTOL aircrafts because at zero speed there is no rotating momentum.
The three spool RB199 in the Tornado jets has also counter rotating shafts by the way.


text:
Does the cockpit have an alarm to warn about fuel leakage?
Is there any indication to the crew of the aircraft about any potential leakage of fuel from any part of the plane?

There is a fuel gage that you should keep an eye on.

I recall that the flight computer on Airbus / Boeing computes the fuel quantity that should be using engine fuel flow. If the calculated fuel and actual fuel differs too much, it will show an amber alert.

On every light aircraft I know of the fuel leak detectors are the pilots' eyes and nose. Fuel stinks, even a small leak can make things very unpleasant in the cockpit (trust me from first hand experience). A loose fuel cap can empty a tank in minutes due to suction, and that's visible to the pilot. Fuel dripping from the airplane is another visible clue.

@GdD That's true in many cases, but at night or in IMC you probably won't be able to see fuel leaking. Of course your nose should still work :-)

My nose seems to work at night, YMMV though ;)

@kevin: Does the avionics knows what is the actual fuel quantity inside the tanks, or just how much is going to the engines (actual fuel flow rate)? Because the actual fuel flow rate will not be impacted by a leak upstream the FF metering unit, at least not before the tanks are empty...

@mins, it knows both, but if the leak is near the engine comparing these still might not detect it, so in the wake of Air Transat 236 incident comparison against flight plan was added.

@JanHudec: Thanks for the info, and the reference to the Transat 236 which I didn't know. Checking in the A320 FCOM, I actually found this: "The FOI system installed on Airbus aircraft use probes to measure the quantity of fuel in the different fuel tanks. Each FOI probe consists of two fixed concentric tubes which form the plates of a capacitor.". So your comment seems to be an answer for Airbus a/c.

As this question stands, I think that it is too broad, because there are many different kinds of airplanes with different capabilities, and the answer will be different for many of them.  Please narrow the scope down to a specific type or group of aircraft so that it can be answered.

Gimly glider was not caused by fuel leakage. The cause was mistake between metric and imperical unit used.

Does the ground crew get the alarm of low fuel too?

In a typical trainer aircraft (in this case, a C172), you have an annunciator panel that includes a "Low Fuel" light:

These lights only come on when fuel is low if the lights are functioning properly. This panel is located prominently, and is easy to notice. This is only one aircraft, but when you consider it is an entry level aircraft that is less sophisticated, it gives a baseline idea of what you could expect to see.
Also, there are fuel gauges displayed with all engine instruments that will indicate accordingly. While this isn't a leak detector, pilots are trained to monitor all instruments regularly and cross check the values - which is how a leak is identified in aircraft with low amounts of technology.
It is worth noting that checking the "LOW FUEL" annunciator is off is a regular checklist item that occurs before you start your actual flight.
As GdD mentions in the comments - you are taught not to rely on any one instrument indication, and panel lights are no exception.

What typical trainer are you talking about? I've been in 1 light aircraft which had a low fuel enunciator, and I was told not to depend on it!

Gah - I put the aircraft type as part of the image description - I'll add it to the answer itself.

While this indicates you get short of fuel, this doesn't indicate if a leak is currently occurring.

@mins - you are correct. I suppose I was assuming that the pilot would interpret the indications of the fuel level indicators (including the panel light) to recognize a leak.

Yes, but this information cannot help act before the tanks are low, which would be possible if a leak alarm existed.

I agree with @mins, this doesn't answer the question.

There is no special indicator or warning about a fuel leak because it would be difficult to automatically detect a fuel leak with any certainty (fuel flow sensors are not super accurate, nor are fuel quantity sensors).
Pilots of small, single tank airplanes must rely on reading the fuel quantity indicators periodically and "knowing" how much fuel should be left and how much is actually left. Big discrepancies indicate a possible fuel leak.
Pilots of bigger, multi-tank airplanes check for (and, on more sophisticated airplanes, are warned about) fuel imbalances between tanks. A fuel imbalance again, indicates a fuel leak as it is very very unlikely that the same quantity has leaked from both tanks.
You can find further info about fuel leak detection on Airbus airplanes here.

authors: المرجع النادر


text:
Can any military aircraft (only fighter jets not the military cargo ones) be stopped using Arresting Gear technique?
I know in mostly all of the seaborne landings on the Naval warships is done using Arresting Gear and there are some land based airfields also which used arrested gear methodology but do any aircraft specifically military ones (just the fighter jets)can be stopped using this or there is something specific feature in an aircraft which makes it Arresting gear compatible?

I think your question is the wrong one.  Can you imagine how large the gear would have to be to stop a C5 Galaxy?  Wouldn't it be better as "why do some military aircraft have arrester gear and some do not"?

C5 is actually a military cargo which can not be landed on naval warships also. So, my questions is specifically for fighter jets etc.. I have edited the question also.

@NitinG It's not the type of aircraft that's the problem; there are cargo planes that land on carriers (e.g. C-2). Carriers don't just have fighters, because they have other things they need as well.

On the flipside, there is a special part that's needed to use arresting gear; most fighters have it (even if they can't land on a carrier), but it's a special part. (incidentally, land fighters generally can't land on a carrier, so that's no reason to not use a tailhook on a cargo plane; the C-130 has been landed on a carrier with no tailhook at all, just excellent short-field performance. Arresting behavior is independent of plane type; size is relevant but not the only thing that matters, type doesn't matter at all.)

In order to engage the usual arresting gear (cables) on an aircraft carrier,
it helps to have a tail hook.
There are other forms of arresting gear that do not require a tail hook.
Presumably the main limitations on an aircraft using them are the size, weight,
and landing speed of the aircraft.
These devices do not seem to be designed for as frequent use as
the arresting cables of a carrier.
Followup: As I found out on looking into this further,
many (most? all?) present-day land-based fighter jets,
not just Navy or Marines jets, have tail hooks as well.
However, at least according to this blog entry,
tail hooks of the types of aircraft used by the US Air Force (and some other
air forces) are not designed to work with aircraft carrier arresting cables
but only with land-based systems (which suspend the cables higher than the
aircraft carrier systems do).
As mentioned in a comment (and in the Wikipedia article on "Arresting gear" linked above), barrier nets can serve as arresting gear.
Various publications (for example the Supplement Alaska, pp. 10–11,
accessed 22-Jan-2015)
list numerous kinds of gear, including cables designed to engage tail hooks, woven straps designed to be torn by tail hooks (one-time use only!), nets designed to engage struts, nets designed to engage wings, nets combined with cables, and engineered material arresting systems (EMAS) that absorb energy as they are crushed under the wheels of the aircraft. I have not yet found any citations of EMAS being used by military aircraft, but its main limitation of seems to be landing speed rather than aircraft weight or size, as an FAA fact sheet on EMAS dated December 12, 2014 credits these systems with stopping nine aircraft that overran runways in separate incidents, including a Boeing 747, an MD-11, and an Airbus A320.

Without a specific reference to an arresting barricade, this answer is incomplete

@raptortech97 I've added an explicit list of other systems. The research was engaging (pun intended). It also appears I was under a mistaken impression about the presence of tail hooks on USAF fighters and other such aircraft, so I've attempted to correct that.


text:
Is it possible to do a barrel roll in a large aircraft like a B737?
Can the bigger planes like B737 do barrel rolling kind of stunts like fighter jets do? I'm sure if it's full of passengers or cargo it would be very difficult and dangerous, but is it possible with an empty aircraft?
I remember an All Nippon Airways incident where the co-pilot did a 131 deg roll by mistake but I don't know if a complete barrel roll is actually possible in large planes?

Well, it happened once: http://youtu.be/AaA7kPfC5Hk. That was a much older plane, though.

Big airplane implies big inertia (including roll-wise inertia) and such airplanes are not design to perform such maneuvers. Even if it is not impossible, it must be difficult.

Hmm... I'm not sure that I'd consider this question a duplicate, since the other one is about actual sustained inverted flight, whereas this one was about barrel rolls. Those are two different questions (with very different answers.) Given sufficient altitude, almost any airplane can do a barrel roll, since it's a 1G maneuver, as the pilot who did it in the video raptortech linked correctly pointed out.

Big helicopters can do it too. https://www.youtube.com/watch?v=MQ7pVjQ5Y5g

A 131-degree roll committed by mistake would be more difficult to safely recover from than a 180-degree roll performed by a knowledgeable and skilled pilot as the intentional first half of a continuing barrel roll.  I bet by the time the plane reached 131 degrees bank the nose was very low and the airspeed was rising rapidly.

Yes, it has been done before in an even larger aircraft, the Boeing 367-80, very famously by "Tex" Johnston on August 7, 1955. This aircraft was the prototype for the Model 707.
When a barrel roll maneuver is properly performed the aircraft remains in a positive-G state.  According to the International Aerobatic Club,

The Barrel roll is a combination between a loop and a roll. You complete one loop while completing one roll at the same time. The flight path during a barrel roll has the shape of a horizontal cork screw. Imagine a big barrel, with the airplanes wheels rolling along the inside of the barrel in a cork screw path. During a barrel roll, the pilot always experiences positive Gs. The maximum is about 2.5 to 3 G. The minimum about 0.5 G.

Yes, and its happened at least once in a -80 which eventually became the 707.
It became so legendary that rumor has it

Boeing Chief Test Pilot John Cashman stated that just before he piloted the maiden flight of the Boeing 777 on June 12, 1994, his last instructions from then-Boeing President Phil Condit were "No rolls."

LOL That's a shame. I'd love to see a video of a 777 executing a barrel roll.

Yes, it is possible. We experienced this possibility using a simulator on a 747/400.
You have to take a very high angle at the beginning like 25° nose up to avoid exiting with an important nose down and overspeed.
Of course doing it with passenger is stricly forbiden.

That would certainly liven up a boring flight...

What if you get a majority of the passengers to vote in favour first? :)

@Wossname The minority would sue, and likely win.

Simulator models don't usually include data for maneuvers outside the aircraft's certified envelope. So I would be extremely wary of concluding from a sim maneuver what is possible on the real aircraft, never mid legal. B747 limitations are 2.5G positive, as far as I know.

My friend in high school claimed his father had a guy do it during a 747 (might have been another large model) training flight on him. He was fairly pissed.

Here is a video of a stolen Q400 Dash-8 doing a barrel roll.  That's not quite as large as a B737, but I think it's close enough for the purposes of the question, especially given it was accomplished by an amateur. It's fair to assume that a trained pilot could do at least as well in a much larger plane--if they were willing to risk their job/license.

My friend claimed his dad had a guy do it to him during the training exercises. He was pissed. Believe it was a 747. No idea if he eventually got his license this was over twenty years ago.

Yes it is possible.  It happen in a Brazilian plane hijack in September 1988 on flight VSP375 from Vasp flight company. . The plane was a Boeing 737-317 (# 23176, serial #Boeing 1213, Brazilian register PP-SNT).  The hijacker wanted to throw the plane on Brazilian president official home , the pilot tried the maneuver and succeed with 105 passengers onboard.

Can you add a link to some source describing this, just to make the answer more complete?  It is not as well known as the famous roll in the 367-80.

Yes, it is possible.
This is a video of Captain Brian Walpole, chief pilot of the Concorde fleet recounting how both he and a French test pilot flew barrel rolls in the Concorde during a test flight.


text:
What has been done to provide security for remote wireless updating of in-aircraft software components?
Is it true that in the latest generation of Boeing aircraft some software components / Electronic Flight Bag data can be pushed to the flight remotely via a Wireless link?
If so, what sort of safeguards are in place to preserve the integrity of such an update? How does the aircraft authenticate that the data sent is indeed legitimate?
I'm curious to know the details. e.g. Are these conventional Ethernet networks over which the update gets pushed at the airport end? Is the final wireless transmission done by a COTS WiFi router? Does the update travel via the internet from (say) Boeing till the airport?

"I'm curious to know the details": Not sure the designers want to satisfy your curiosity and help you start searching for the possible weaknesses. Even if hiding things is not a security, it sometime helps.

@mins Oh, I'm pretty sure the designers don't want to. But others often can. :) Peter Kampf did to some extent already. There might be other knowledgeably sources like him. It is amazing how much you can glean from anecdotal sources sometimes.

@mins Yes, I admit hiding things can sometimes help, but when it comes to something as critical as an aircraft the right way to do things IMO is to have better strategies than obscurity. You want to be more sure than PIAs & NDAs to protect your channels. I think security through obscurity is OK for things like toilet master lock opening buttons e.g. http://aviation.stackexchange.com/questions/24054/could-the-cabin-crew-open-the-lavatory-door-from-outside-when-someone-locked-the but not for critical stuff like avionics software updates.

@mins One key issue is how the aircraft system verifies that the patch it is getting over the air is a valid, un-tampered one? Is there a hardware based key verification module in the aircraft?

Also, holes are constantly being found in home / enterprise network setups. But then again, typically none of these domains are designed to the exacting standards of reliability that we have incorporated into aircraft by regulation.

@mins To put it another way: Whether it makes sense for the designers to rely on obscurity for security is their call. But OTOH flyers / pilots / technologists can still have an interest in knowing how the system works to be able to independently assess it without having to have blind faith in the design.

I understand, but you miss the point of how security and authenticity and eavesdropping are managed in general. Crypto keys are not secure if you are able to get a large computing power, this is why their length is increased from time to time. Just consider this fact and how it is similar to obscurity. The only way to be sure to detect data tampering by a "man in the middle" is, to my best knowledge, quantum links, and be sure it won't be used in the aviation industry in the short term.

@mins Isn't something like a 2048 bit key reasonably secure even against the likes of a nationally owned supercomputer? By most estimates against any sort of scaling in CPUs, save quantum computing?

@mins I thought Man In The Middle Attacks can be 100% prevented by previous transfer of a key via a secure channel. e.g. If the hardware key was transferred from Boeing to the aircraft owner (say) by hand then the aircraft can be secure that and data update signed by that key is secure?

Any example I could give is a known threat (hence it has countermeasures) that may give a false feeling of how much we are in control. Anyway ARP poisoning is an example of making ciphering useless. Also you may remember the goto fail bug that left Apple devices exposed to SSL easy hacking for years. If you want more facts, then this has to be in a chat session, to comply with SE guidelines we already broke.

@curious_cat Amount of simple operations that is safely out of reach for all humanity? on the [security.se] SE might be of interest.

@MichaelKjörling: While a huge power seems required to break encryption, in practice keys are not distributed equally for some pseudo-random number generators. Weaknesses exist in the crypto standards. Theoretical security is one thing, practical implementation is another.

@curious_cat wrote "If the hardware key was transferred from Boeing to the aircraft owner by hand ...."  They don't even need to do that.  Boeing built the plane.  They can install the certificate in the computer while they're still building the plane!  They know what certificate is installed, and can use a unique, known certificate for every plane in existence.

Boeing claims in this document:

The 787 uses wireless (802.11 b/g) technology while at the gate to
  transfer maintenance data and update the Electronic Flight Bag (EFB).
  This is referred to as “e- Enabling” and it will allow airlines flying
  the 787 to operate more efficiently.

So, yes, they use a regular WLAN connection for transmitting data while on the ground.
The document continues:

Upon weight on wheels the 787 attempts to authenticate specific ground
  access points. Information regarding the key configuration
  requirements and how the 787 authentication occurs requires the
  signing of a PIA.

I interpret PIA as Proprietary Information Agreement, a form of secrecy agreement. Boeing uses security through obscurity to safeguard the integrity of the connection.
If you want to know more, Boeing advises to contact 787terminalwireless@boeing.com. However, I doubt they will talk to you or me unless we are airport representatives with a credible need to know.

Ha! That's the worst kind of security & is exactly what I feared. I'm hoping that the grapevine will reveal more details. There must be a ton of people at airports exposed to this which unfortunately makes it neither secure nor obscure.

You only have to review the Stuxnet saga to see how security through obscurity works in practice for mission critical equipment.

@curious_cat I don't see what is wrong with adding security through obscurity to an otherwise secured system.

@DeltaLima, security through obscurity does not stop the serious attacks, but it stops the less serious ones and the white hats and security researchers, but you want those to test the system to warn you about vulnerabilities before the serious attackers abuse them.

@JanHudec That is a fair point.

@JanHudec Well said!

Don't they have to reveal some of this info. to the FAA for certification? And once done, they ought to be more accessible for the rest of us. Actually, how much of this comes under the ambit of FAA certification? Where's the boundary line between systems that need FAA certification and ones that don't? e.g. Would the WLAN and specific protocols used to push updates and verify updates be subject to FAA oversight?

@curious_cat "exactly what I feared".  What is your fear?  You seem to assume that some obscurity is the only security, which it is not.  It is simply yet another layer which, as others have pointed out, is just a case of "why make it easier".  They are not using open internet and standard protocols here.  It's not a system a script kiddie or anonymous no-friends cyber-psychopath can just break into.  Also, they key, safety critical systems cannot be updated "over the air".  They have replaceable cards and in some cases, plugin updates which require physical access.

@Simon How to distribute software securely over potentially hostile networks is a fairly simple, well understood problem with multiple effective solutions in commercial (and non-commercial!) use. If the update channel were secure in such a way, its details could be published in the New York Times and updates would remain secure. The very existence of the PIA calls the overall security of the system into question.

@MichaelHampton I know, I'm a software engineer :), and I use government secure PIAs all the time.  You contradict yourself by saying that there are multiple effective solutions for comms in hostile environments (and I agree), but then question the system used since it involves a PIA.  The key point is that the aircraft "firmware" cannot be update using this system, which can only be achieved with physical access.  The insecurity of using iPads and the like for flight bags is already established.

In theory, they are just transmitting data, not code updates to the aircraft. So, the worst-case scenario would be that somebody would replace a chart with fake chart or something like that, which would be a nuisance but should not affect the safety of the flight.
As a matter of routine, any radio link should be considered an inherently insecure mode of communication, because it is open, unlike wire connections which can be physically isolated and secured against intercept.
Of course, if there is a bug or weakness in their system somewhere, it might be possible for a hacker to "pierce the veil" and somehow modify a live system. Whether that is possible or not depends entirely on the system architecture and implementation details of the software in question. Without knowing this (highly proprietary) information it is impossible to know whether such a weakness exists.
The official guidance on practices is contained in various FAA circulars, notably FAA advisory circular (AC) 20-153.
Here is a paper describing the general security situation: FAA Aircraft Systems Security.
Only the 787 is able to load actual flight system modules remotely, I believe, although as mentioned above many aircraft can wirelessly retrieve flight bag info. How Boeing verifies that a module is valid before loading I do not know, but in all likelihood they use a cryptographic certificate. If this is the case, then a hacker would either have to steal the cryptographic keys from Boeing, or find a bug in how they were using the certificate, in able to introduce a false module to the aircraft.

Are you sure no code updates are pushed over wireless? From what I remember the Wireless link is also proposed to be used to push Loadable Software Airplane Parts. That sounds like code.

Wired connections are no more secure than wireless if you are determined.  There exists equipment to intercept traffic on wired networks with no physical access from tens of metres away.  Really secure installations use specially designed buildings to mitigate this.

@abelenky You would use encryption to prevent industrial espionage, among other things.


text:
Does the "follow" instruction give an aircraft clearance to turn onto a taxiway and/or cross a runway?
Consider this ATC instruction from a ground controller:

N123, continue down Alpha, turn right at Kilo, hold short of RWY36.
N456, follow N123.

Does this instruction mean N456 can turn right at Kilo without further ATC instruction?
What about:

N123, continue down Alpha, turn right at Kilo, cross RWY36.
N456, follow N123.

Must N456 hold short of RWY36 and await clearance to cross the runway?

The controller is required to give explicit clearance for crossing a runway.  "Follow" is not clearance to cross a runway.
From the ATC phraseology:

7-3-2
d. When an aircraft/vehicle is instructed to “follow” traffic and
requires a runway crossing, issue a runway crossing clearance in
addition to the follow instructions and/or hold short instructions, as
applicable.
EXAMPLE−
“Follow (traffic), cross Runway Two−Seven Right.”
or
“Follow (traffic), cross Runway Two Seven−Right, hold short Runway
Two−Seven Left.”

The AIM doesn't spell this out as explicitly.  AIM 4-3-18-5

A clearance must be obtained prior to crossing any runway. ATC will issue an explicit clearance for all runway crossings.

The explicit requirement for runway crossings is somewhat new (2010 I think) and was the more interesting part.  I neglected the question about crossing taxiways:

Does this [follow...] instruction mean N456 can turn right at Kilo without further ATC instruction?

Yes.  Also from 7-3-2

a. Note 1. The absence of holding instructions authorizes an
aircraft/vehicle to cross all taxiways that intersect the taxi
route.

That said, I would not expect the only instruction from the controller to be "follow (traffic)".  I would expect to be cleared to a specific destination as well.

"N456, taxi to 36.  Follow N123"

In practice, the instruction is usually "N456, follow the Cessna ahead, cross rwy 36" or "N456, follow the Cessna ahead, hold short rwy 36 on A".  The controllers tend to be very clear about "cross" & "hold short" instructions, not leaving you guessing. In rare cases when they don't specify, it's usually when the lead aircraft has been cleared all the way to where-ever, including clearance to cross the intervening runway, and that's what the controller intends for the trail aircraft to do to. N456 absolutely needs to confirm that, though. NEVER assume you're cleared across a runway!